1. Field of the Invention
The present invention generally relates to a memory circuit, and particularly relates to a nonvolatile memory circuit which is capable of retaining stored data in the absence of a power supply voltage.
2. Description of the Related Art
Nonvolatile semiconductor memory devices, which can retain stored data even when power is turned off, conventionally include flash EEPROM employing a floating gate structure, FeRAM employing a ferroelectric film, MRAMs employing a ferromagnetic film, etc. There is a new type of nonvolatile semiconductor memory device called PermSRAM. PermSRAM uses a pair of MIS (metal-insulating film-semiconductor) transistors as a nonvolatile memory cell (i.e., the basic unit of data storage). The MIS transistors used as a nonvolatile memory cell in PermSRAM have the same structure as ordinary MIS transistors used for conventional transistor functions (e.g., switching function), and do not require a special structure such as a floating gate or a special material such as a ferroelectric material or ferromagnetic material. The absence of such a special structure and special material offers an advantage in cost reduction. PermSRAM was initially disclosed in PCT/JP2003/016143, which was filed on Dec. 17, 2003, the entire contents of which are hereby incorporated by reference.
The pair of MIS transistors used as a nonvolatile memory cell in PermSRAM are configured to selectively experience an irreversible hot-carrier effect on purpose for storage of one-bit data. Here, the irreversible hot-carrier effect refers to the injection of electrons into an oxide film of a selected transistor, which creates a shift in the threshold voltage of this transistor A difference in the transistor characteristics (i.e., difference in the threshold voltage) between the two MIS transistors caused by the hot-carrier effect represents one-bit data “0” or “1”. Such a difference may be detected as a difference in the ON current between the two MIS transistors by using a sensing circuit such as a one-bit static memory circuit (latch) coupled to the MIS transistor pair.
There are many applications in which PermSRAM can be effectively utilized to provide cost advantages over other types of memories.